The present invention generally relates to apparatus and methods of optimally utilizing power converters and, more specifically, to apparatus and methods of optimally utilizing power converters as part of an engine start system.
Aircraft engines require a start system to generate the mechanical torque required to bring the engine from a stopped state up to a target speed, at which point the engine is considered to have transitioned to a running state. Modern aircraft, such as More Electric Aircraft, have engine start systems that may include an AC power generator, a power converter, and an input power source. For these modern aircraft, during an engine start mode, the generator is operated as a torque-producing motor that uses power supplied at varying voltage and frequency by the power converter. The power converter is supplied with input power from an input power source. When the engine has transitioned from the stopped to the running state, the power converter is disconnected as an AC power supply. In the running state the engine produces mechanical torque which is transformed to AC power by the generator.
Utilization of the power converter as an AC power supply that receives input power can cause the power converter temperature to increase to a level unsafe for continued operation. This increase in temperature necessitates power converter thermal protections which may include operation time limits. Power converters have a rating that specifies limits for their continued operation followed by a minimum wait period. In an existing engine start application for an aircraft, the maximum duration of power converter operation is typically 135 seconds followed by a minimum wait period of 15 minutes. According to these limits, when the power converter has been utilized for the maximum rated duration of 135 seconds, the power converter should not be used for at least the next 15 minutes. The power converter rating serves as a guide to operating the power converter within the power converter operation limits. The power converter operation limits define the onset of thermal damage to the power converter. During engine start mode, the power converter is utilized continuously with input power during each start attempt. Typically current engine start systems do not track the actual operation time of the power converter. The start duty cycle normally expected consists of a single successful start attempt typically having a 40 second duration. The wait period is required so that the temperature of the power converter is reduced. The maximum start duty cycle is defined as the maximum number of consecutive start attempts estimated to occur within the power converter maximum utilization duration followed by the minimum wait period. In an existing engine start application for an aircraft, the maximum number of consecutive start attempts may be three. Thus after three start attempts, the engine start system is required to wait in an idle state for at least 15 minutes before another engine start attempt can be made.
Duty cycle abuse is defined as exceeding the maximum number of start attempts within the fixed maximum start duty cycle duration. When there are unsuccessful starts, it can be expected that the engine start system will encounter duty cycle abuse when the operator attempts multiple starts. In an existing engine start application for an aircraft, duty cycle abuse may be defined as four start attempts without pause, which are caused by consecutive unsuccessful starts. Duty cycle abuse avoidance is used in current engine start systems as a way to prevent exceeding the power converter rating.
Engine start systems that use a maximum start duty cycle based on a fixed number of start attempts are prone to unnecessarily long wait periods. This arises because the actual power converter utilization during the start duty cycle may be less than the estimated utilization. In addition, preventing duty cycle abuse does not necessarily prevent exceeding the power converter rating. This can arise if the actual power converter utilization during the start duty cycle is more than the estimated utilization. Current engine start system maximum start duty cycles can therefore introduce unnecessarily long wait periods while also allowing the power converter rating to be exceeded.
As can be seen, there is a need for a more accurate and efficient power converter utilization method in engine start systems. Additionally there is a need for a power converter utilization method that provides better protection against exceeding the power converter thermal rating. This invention can be extended for any More Electric Vehicle applications, which utilize an electric engine start system.